1. Field of the Invention
The present invention relates to a method of and an apparatus for electrostatically spray-coating a workpiece with electrically-conductive paint directly subjected to a high voltage by using a rotatable atomizing type spray gun.
2. Description of the Related Art
As a method of applying a high voltage to electrically-conductive paint so as to electrostatically spray-coat an object or work such as a vehicle body with the paint, there has heretofore been known a voltage blocking method, for example. According to this method, the paint is first introduced into an intermediate reservoir electrically insulated from ground potential. Thereafter, the paint is supplied via a paint line to a spray gun which is at high potential from the intermediate reservoir, thereby electrostatically spray-coating the workpiece with the paint.
When a rotatable atomizing type spray gun having a rotatable cup (corresponding to a rotatable atomizing head) is used as the spray gun in the above method, water-based paint, if used as the electrically-conductive paint, tends to adhere to the rotatable cup so as to form a dry film thereon. This dry film then separates from the rotatable cup and adheres to the workpiece being coated, thereby causing a painting failure.
As has been disclosed in Japanese Patent Application Publication No. 2-57994, for example, there has been known a method of intermittently supplying cleaning liquid to a rotatable cup so as to clean or wash the rotatable cup while a workpiece is being electrostatically spray-coated with paint.
In the above disclosure, however, the cleaning liquid has been supplied intermittently to the rotatable cup. Therefore, an undesired flow of electricity through the cleaning liquid tends to occur when a high voltage is directly applied to the water-based paint to carry out an electrostatic spray coating or painting process. Accordingly, the voltage applied to the water-based paint is unstable, and the workpiece which has been electrostatically spray-coated with the paint becomes inferior in quality owing to impairment in the efficiency of the application of the paint to the workpiece. In addition, the voltage to be applied to the paint is greatly reduced, so that an electrostatic spray coating process cannot be carried out.
Further, in the above disclosure, the dry film produced by the paint is prevented from being applied to and deposited on an inner peripheral wall of the rotatable cup by coupling a paint feed pipe and a water feed pipe for washing to the rotatable cup and supplying cleaning liquid to the rotatable cup from the water feed pipe by a water feed valve.
However, a tip portion and an outer surface of the water feed pipe cannot be cleaned, and the cleaning liquid is supplied intermittently to the rotatable cup. Therefore, water or moisture on the tip portion and the outer surface of the water feed pipe evaporates to dryness while the supply of the cleaning liquid to the rotatable cup is being stopped, thus allowing the formation of solid materials such as a dry film produced by the water-based paint, etc., thereby causing a problem in that these solid materials are then applied to the workpiece. As a result, continuous painting cannot be carried out using the water-based paint, and items to be electrostatically spray-coated with the paint cannot be mass-produced.